A taxonomical approach to gravimetric dosing
The proportioning of bulk materials is a key functional task within the overall process of cement manufacturing and all related transport and logistic systems. Even if many different system architectures are proposed throughout the last decades, there is no systematic classification available, which allows a strategic decision for a specific methodology. The article (part 1) talks about one of the success stories of a product – ODM-WeighTUBE®, a tubular gravimetric high-precision dosing system from DI MATTEO Group.
The cement manufacturing process and all related transport and logistic systems are driven by strict requirements regarding the accurate proportioning of bulk materials from various sources. The corresponding integration of adequate parts of equipment requires the systematic selection of dosing and/or metering devices or scales and the subsequent consideration of the necessary weighing technologies. If the complete chain from the quarry and subsequent handling of raw materials to the dispatch of the final product in bags or as bulk cement is analysed, it is obvious that the applied dosing equipment needs to be able to handle a great variety of different types of bulk materials in terms of their general appearance (powdery, granular, flaky, fibrous etc.), their followability (free-flowing, cohesive, etc.), their granularity (size of smallest and biggest 2D or 3D particles) and their general bulk material properties (e.g. density, humidity, etc.).
Within this context, it is also important to consider the long-term stability of the aforementioned characteristics, which is often underestimated during the design phase of bulk material handling systems, especially if there is a lack of experience with the utilisation of novel materials, e.g. during the introduction phase of new alternative or secondary fuels. Therefore, it is essential to start the systematic selection of adequate dosing equipment with a holistic approach towards the exact characterisation of the bulk material by means of mechanical, chemical and other relevant evaluation methods. It is also noteworthy, that the actual process requirements of the proportioning process can also be completely different, depending on where the system shall be installed within the complete manufacturing and logistic process chain. Very important within this context are quite often also legal requirements, which need to be fulfilled due to quite strict international standards and quality management procedures.
If all above mentioned boundary conditions are well defined, it is still not guaranteed, that the actual task of choosing and integrating the optimal corresponding dosing equipment can be completed successfully, since it is observable that the decision for one or other solution is not following clear rational guidelines, but more fuzzy decision factors. A typical example for such a misconception during the planning and installation of dosing equipment is the transfer of positive or negative operational experiences with a particular piece of equipment from a quite specific field of application to a completely different scenario. Such an ill-posed transfer of experience is a typical trap during all conceptual phases of modern engineering, since it is contrary to a logical and rational selection of the optimal equipment.
DI MATTEO developed since its establishment in 1961 an immense experience with the implementation of all kinds of proportioning devices for a great variety of bulk materials in numerous different application fields. From the efficient feeding of alternative fuels (AFs) (e.g. RDF, shredded tyres, sewage sludge, etc.), over the classical dosing of raw materials (limestone, clay, sand, iron ore, etc.) to the implementation of all kinds of weighing hoppers and silos, all types of possible projects were already successfully engineered and realised in cement plants, steel factories and power plants all over the world. Furthermore, the company acted also as a driver of innovation within this field, with the successful introduction of the award-winning patented new tubular gravimetric dosing system ODM-WeighTUBE® (the development of the modular and comprehensive ODM-GravitAS control platform and software library, as introduced in (see Figure 1).
In order to give a clear guideline which proportioning system or method is a preferable solution within different application scenarios, this article provides an introduction to a framework for the systematic classification and description of different approaches to dosing and metering of bulk materials, which was developed by DI MATTEO. With the integration of the classification system a corresponding taxonomy is proposed.
The main aspects in order to evaluate the performance of a weighing system can be summarised as follows:
The accuracy of a dosing or weighing system is the degree of closeness of measurements of a quantity (e.g. massflow) to that quantity's true value.
The stability of a bulk material dosing unit is typically defined as a time interval in which the accuracy definitions associated to the system can be guaranteed without any manual intervention.
The availability of a dosing unit can be defined as the typical ratio between the time where the system is operatable and the total working time in a defined period of time.
For this reason, section 2 introduces a general classification framework for metering and dosing units. This classification is then used in section 3 to differentiate the typical range of products within the field of bulk material proportioning. One of the latest developments within the field of bulk material proportioning is the introduction of the award-winning ODM-WeighTUBE®, which is presented in section 4. Finally section 5 summarises the article and provides a conclusion to the topic.
General classification of bulk-material dosing devices
The base for a classification of different dosing and proportioning devices for bulk materials is the former definition of the functional entities of those machines. For this it is reasonable to distinguish three main functional elements of typical dosing devices: (i) measuring, (ii) conveying and (iii) controlling as proposed in . The combination of either any two or all three of these basic elements defines the specific character of the equipment, as shown in Figure 2.
Here, it is possible to name six different classes of machines as a general taxonomy for the proportioning of bulk materials. The exact definition and corresponding aspects are summarised within Table 1.
All of these machines are justified within their specific field of application and there is no suitable piece of equipment which can meet all technological and economic requirements of every possible field of application. If for example a continuous flow of material is required by the process (e.g. supply of alternative fuels), there is no possibility to utilise an indirect dosing setup, such as a loss-in-weight system, even if other aspects indicate it.
Therefore DI MATTEO developed and offers products from all the mentioned categories and within the next section, some of the available products will be classified by using the aforementioned framework, in order to get a clear understanding of the physical realisation of the introduced categories.
Typical proportioning equipment
There is a great variety of available products in the field of bulk material dosing and proportioning and within this section some prominent examples are used to clarify the taxonomy as introduced in section 2. For this the six categories, as introduced in Table 1, are used to provide a classification of the product range from DI MATTEO.
Pure Measuring Devices – Silo Scales and Weighing Hoppers
Typical examples for pure measuring devices are weighing hoppers and silo scales, which are mainly used in order to determine the actual filling in terms of weight m [kg] and/or, for bulk material with constant density and humidity, also the volume V [m³]. The mass of the material is measured by means of a set of load cells, corresponding mechanical mounting modules for effective force transmission (see Figure 3) and a corresponding evaluation unit
A proper proportioning of the material within the silo or hopper is only possible, if the setup is complemented with a controllable discharge system (e.g. a screw discharge bottom – ODM-ScrewDOS). Such a setup would be an indirect dosing system, where the following section provides an example.
Indirect Dosing – Differential dosing setups
The indirect dosing setup consists typically of a silo scale or weighing hopper, which is equipped with a controllable discharge system, as shown in the general principle illustrated in Figure 4. Here a screw is used to discharge a hopper, which is placed on a set of load cells, which acquire the actual material weight m [kg], within the hopper. By using the loss-in-weight principle the actual mass flow [kg/h] can be continuously calculated by taking into consideration the mass difference Dm [kg] within a defined time interval Dt [s] (time derivative of the material weight), according to the following relation.
By an adaption of the screw speed it is therefore possible to achieve a desired mass- or volume flow. However, the main drawback of this setup is the missing possibility to dose the material continuously, since the relation shown above is of course only true, if there is no simultaneous material infeed to the hopper. This means, that the setup is only suitable for batch-type dosing tasks, such as batch mixing or filling.
ABOUT THE AUTHORS:
Prof. Dr Dominik Aufderheide; South Westphalia University of Applied Science, Soest, Germany
Dr Luigi, DI MATTEO Group, Beckum, Germany.
DI MATTEO Group is a family-run business that was founded in 1968 by Orlando Di Matteo. The Group specialises in handling of bulk materials like waste, wood and biomass, etc. for process industry. Part II to be continued in the next issue.